The invention relates to a drafting unit for a spinning machine having a drafting zone delimited by two pairs of rollers, which are each formed of top rollers and bottom rollers, an apron guide having top aprons and bottom aprons being located in this drafting zone, the top apron being wound around the top roller and the bottom apron being wound around the bottom roller of the preceding pair of rollers, and the aprons, in each case, being guided around reflecting guides in proximity of the nip line of the following pair of rollers.
Drafting units of the initially mentioned type are used in spinning machines of a variety of types, particularly in ring spinning machines. With respect to their construction, a basic rule has developed according to which it is provided that the diameters of all rollers are maintained to be essentially identical.
In modern spinning machines and particularly in new technologies, such as pneumatic false-twist spinning or the only pneumatic prestrengthening of yarn components which are to be later twisted together, significantly higher working speeds are obtained than during ring spinning, so that the working conditions of the drafting units change also. It was found, for example (EP-B No. 01 07 828) that the air currents generated by the top and bottom rollers running at high circumferential speeds interfere with the order of fibers in the sliver. It is therefore provided in this construction that the top aprons and the bottom aprons, in the area not carrying any fibers, are guided as closely as possible to the circumference of the top rollers and the bottom rollers, so that the air currents are deflected already in front of the area of the nip line in axial direction of the rollers and cannot interfere with the order of the fibers.
An object of the invention is to provide a drafting unit of the initially mentioned type which, in particular, is suitable for the processing of short-staple fiber material at high speeds.
This object is achieved according to preferred embodiments of the invention by providing that the diameter of at least the bottom roller of the following pair of rollers is smaller than the diameter of the bottom roller of the preceding pair of rollers.
By means of this construction, it is achieved that the distance can be reduced of the nip area between the top apron and the bottom apron from the nip line of the following pair of rollers which, as a rule, will be the pair of delivery rollers. Thus, the area is shortened in which the fibers are not guided. In addition, it is achieved by means of the reduction of the diameter that the air current caused by the respective roller is reduced, the strength of this air current depending on the diameter.
In a further development of preferred embodiments of the invention, it is provided that the bottom roller of the following pair of rollers is constructed as an over-mounted shaft piece which is connected to a drive. By means of this construction, which is known in principle from German Published Examined Application (DE-A) No. 33 19 559, it is possible to fix the diameter irrespective of the normal restrictions. Specifically, in the previous constructions, in which the bottom roller was constructed as a cylinder (composed of several parts) extending through in longitudinal direction of the machine, it was not possible to fall below a certain diameter. The reason is that care had to be taken that the occurring torsional tensions would not result in excessive torsional deformations. By means of the division into individual shaft pieces, it is possible to clearly differentiate and make the effective diameter smaller than the previous minimum diameter of approximately 25 mm, without the occurrence of excessive torsional strains.
The pair of rollers which, in the context of the present invention, is called the "following pair of rollers", will, as a rule, be the pair of delivery rollers. However, embodiments are also contemplated wherein several drafting zones are provided which contain apron guides and which are then each delimited by a pair of rollers. In order to be able to move the fiber carrying area of these apron guides as closely as possible to the nip line of the following pair of rollers, corresponding adaptations of diameter may be carried out.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.